This invention relates to a method for the production of sodium perborate hydrate granulates with an available oxygen content (Oa) of 14 to 18% by weight. More particularly, the invention relates to a method of producing sodium perborate monohydrate granulate with an available oxygen content of 15 to 16% by weight; with a bulk weight from 0.6 to 1.2 kg/dm.sup.3 ; an abrasion index according to ISO test 5937 of not more than 10%; and a rate of dissolution below 1.5 minutes for 1 gram of granulate with granularity in the range of 0.2 to 0.8 mm in 500 cm.sup.3 water at 15.degree. C. The method of the invention includes compressing and granulating a sodium perborate hydrate starting material with a bulk weight lower than that of the granulate product. Another aspect of the invention includes the granulate products with the properties specified above, which are obtained in accordance with the method of the invention.
The term sodium perborate hydrates signifies compounds of the general formula: ##STR1## wherein x is 6 for the product customarily designated in the art as sodium perborate tetrahydrate, and x is 0 for the product with 16% by weight available oxygen (Oa) designated in the art as sodium perborate monohydrate. Perborates known as super-oxidized perborate hydrates with an Oa content above 16% by weight are also known; however, the structure of these super-oxidized perborate hydrates does not appear to be well established. Therefore, the granulates with an available oxygen content (Oa) of 14 to 18% by weight, which are produced in accordance with the invention, may include different structures, although sodium perborate monohydrate constitutes the main product.
Sodium perborate hydrates with an available oxygen content in the range of 14 to 18% by weight, especially sodium perborate monohydrate with Oa=15 to 16% by weight, are becoming increasingly more important because of their high available oxygen content, which is greater than that of the tetrahydrate product. The sodium perborate monohydrate product also exhibits a higher rate of dissolution than the tetrahydrate product. In addition, in order to make for low-dust pneumatic conveyance, a high abrasion resistance and as high a bulk weight as possible are required of the perborates. This low-dust characteristic is important to enable the use of the products in so-called detergent volume concentrates.
The production of products with the above-mentioned properties is problematic, because the material properties of abrasion resistance, rate of dissolution and bulk weight are in part inversely dependent upon one another, i.e. creating a product which maximizes one of the properties tends to adversely affect the other properties.
Perborate monohydrate prepared by means of the dehydration of crystalline perborate tetrahydrate does exhibit the desired rate of dissolution, i.e., under 1.5 minutes for 1 gram in 500 cm.sup.3 water at 15.degree. C.; however, this product is not very abrasion-stable and, in addition, only bulk weights below 0.6 kg/dm.sup.3 are available. Maintaining special dehydration conditions can increase the abrasion resistance, but the bulk weight is not significantly increased, cf. DE-PS 22 58 319, DE-PS 24 44 780, EP-A 194,952, EP-A 202,519. The perborate monohydrate granulates obtained according to EP-A 202,518, EP-A 295,950 and EP-A 296,813 contain heterogeneous binding agents such as sodium silicate and carboxylic acid compounds; however, these products do not exhibit the required properties for the granulate described above.
An attempt has also been made to further improve the abrasion resistance of perborate monohydrate by means of moistening and drying, as disclosed in U.S. Pat. No. 4,002,434. However, while the rate of dissolution does remain high and the bulk weight does not change significantly, it has been noted that the abrasion resistance of the resulting products no longer meets current standards. Sodium perborate monohydrate granulates containing zeolites, as disclosed in DE-PS 26 52 488, with a variably adjustable rate of dissolution, abrasion resistance and bulk weight are known; however, the mandatory presence of large amounts of zeolite lowers the Oa content and limits the possibilities for use of this composition.
It is possible, according to the methods of U.S. Pat. No. 4,115,519 and U.S. Pat. No. 4,185,960, to produce abrasion-resistant, granular sodium perborate monohydrate and perborate hydrate with an Oa value above 16% by spraying an aqueous solution of hydrogen peroxide and a solution of sodium metaborate into a fluid-bed dryer containing nuclei with a diameter smaller than the granulate to be produced. An abrasion-resistant granulate with a high bulk weight is produced in this manner; however, this method forms a product wherein the rate of dissolution is reduced considerably compared to perborate monohydrate prepared by dehydration. This unsatisfactory rate of dissolution may be improved somewhat by means of the simultaneous use of a tenside or surface active agent as in the previously mentioned production method, cf. U.S. Pat. No. 4,215,097. But, granulates with a rate of dissolution below 1.5 minutes for 1 gram of the granulate in 500 cm.sup.3 water at 15.degree. C. were not able to be produced in any manner, as shown in the examples in U.S. Pat. No. 4,215,097.
European Patent Application 328,768 teaches another method for the production of sodium perborate granulates with Oa values of preferably 14 to 16% by weight, a high bulk weight, a high rate of dissolution and an abrasion in the range of the marketed products which are to be produced. In this method, essentially amorphous primary particles which are obtained in the absence of nuclei by means of spray drying are supplied immediately upon leaving the drying zone to a granulating device and granulated in the presence of free and/or perborate crystal water. It was pointed out in this document that perborate hydrate granulates with bulk densities of up to 1.0 g/ml can be produced by using a fairly high-compressing granulating device, e.g. a continuously operating mixer or a drum with revolving blades or worms. It was considered to be essential in EP-A 328,768 that the granulation be carried out immediately following the formation of the primary particles by spray drying.
It was determined that commercially available sodium perborate monohydrate obtained by dehydration with a bulk weight of approximately 550 g/dm.sup.3 cannot be satisfactorily compacted using a roller compactor, nor can a granulate be obtained from the dehydrated shell product. The shell product yield is very small, approximately less than 5%, and, in addition, only a slight increase in the bulk weight can be obtained. Furthermore, the tendency of the granulates to adhere to the rollers constitutes an additional problem.